As an example of a conventional sphering apparatus, the sphering apparatus described in Japanese Patent Publication No. 3331491 is shown in FIG. 2. This sphering apparatus is composed of a sphering furnace 10, a cyclone 20, and a bag filter 30.
The sphering furnace 10 (hereinafter referred to as “furnace 10”) melts the raw material in flames, thereby forming particles. The cyclone 20 suctions and classifies the particles formed by the furnace 10, followed by trapping only the particles having a predetermined particle size or more. The bag filter 30 traps the fine particles which are not trapped by the cyclone 20. The particles trapped by the cyclone 20 and the bag filter 30 are separately used or appropriately blended for industrial use.
At the top of the furnace 10, a sphering burner 11 is provided in a vertically downward direction. In the sphering burner 11, the raw material, combustion-assisting gas such as oxygen or air, and fuel gas such as propane gas are supplied. The mixed gas of combustion-assisting gas and fuel gas forms a flame in a downward direction. As the raw material is supplied and melted in the flame, the particles are formed.
The formed particles pass down the furnace 10 and are supplied into the cyclone 20; however, some of the just formed particles occasionally adhere to the inner wall of the furnace 10. The particles adhering to the inner wall form a bulky object, and the surface of this bulky object is heated to a high temperature so that newly formed particles further adhere. When the particles formed in the flame adhere to the inner wall of the furnace 10, the amount of the particles supplied into the cyclone 20 is decreased, so the production efficiency of the product becomes deteriorated. Also, when a bulky object is formed on the inner wall of the furnace 10, the temperature in the furnace 10 is increased, so the amount, the particle size, and so on of the particles formed in the furnace 10 are changed from the predetermined ones, As described above, the adhesion of the particles to the inner wall of the furnace 10 and the formation of a bulky object cause problems.
Therefore, a plurality of adhesion-preventing air-introducing holes 12, 12, . . . is formed in the furnace 10 so as to prevent the particles from adhering to the inner wall of the furnace 10. In FIG. 2, five of the adhesion-preventing air-introducing holes 12, 12, . . . are formed in a vertical row, and a plurality of the vertical rows is formed along the circumferential direction of the furnace 10.
The adhesion preventing air-introducing holes 12, 12, . . . are connected to an air blower 40a (hereinafter referred to as a “blower 40a”) through a manifold 41. The air from the blower 40a flows into the furnace 10 through each of the adhesion preventing air-introducing holes 12, 12, . . . so as to prevent the formed particles from adhering to the inner wall of the furnace 10.
Meanwhile, a viewing window which is not illustrated in FIG. 2 is provided in an upward direction at the bottom of the furnace 10, so the inside of the furnace 10 can be viewed. Accordingly, it is possible to check if the particles adhere to the inner wall of the furnace 10 or not.
At the lower part of the furnace 10, a carrier air-introducing hole 13 and a carrier air-withdrawing hole 14 are formed in opposition to each other. To the carrier air-introducing hole 13, an air blower 40b (hereinafter referred to as a “blower 40b”) supplying carrier air is connected through a first pipe 42.
As described above, the cyclone 20 classifies and traps the particles having a predetermined particle size. In order to maintain a constant classifying condition and an optimal trapping efficiency of the cyclone 20, it is necessary to maintain an air amount Q (hereinafter referred to as “Q”) supplied into the cyclone 20.
The Q is a sum of an adhesion-preventing air amount Qa (hereinafter referred to as “Qa”) and a carrier air amount Qb (hereinafter referred to as “Qb”). In other words, the following equation (1) is established.Qa+Qb=Q  (1)
Therefore, in order to maintain Q constantly, it is necessary to equalize the variation of Qa (hereinafter referred to as “ΔQa”) and the variation of Qb (hereinafter referred to as “ΔQb”). In other words, the following equation (2) should be fulfilled.ΔQa=ΔQb (in which ΔQa, ΔQb>0)  (2)
However, there is the problem in that it is difficult to equalize ΔQa and ΔQb.
Hereinafter, the reasons causing the aforementioned problem are described in detail.
For example, when the particles start to adhere to the inner wall of the furnace 10, in order to prevent this adhesion, the rotational frequency of the blower 40a is increased so as to increase the current Qa to Qa+ΔQa in quantity. Then, the discharging pressure of the blower 40a is increased, and the pressure in the furnace 10 is increased accompanying this. As a result, a pressure difference between the blower 40b and the furnace 10 occurs, and Qb is decreased by this pressure difference. Therefore, in the case of adjusting Qb, it is necessary to take the decreased quantity of Qb into account, which is caused by the pressure difference between the blower 40b and the furnace 10.
Meanwhile, in order to fulfill the equation (2), the rotational frequency of the blower 40b is decreased so as to decrease the current Qb to Qb−ΔQb in quantity. Then, the discharging pressure of the blower 40b is decreased, and the pressure in the furnace 10 is decreased. As a result, a pressure difference between the blower 40a and the furnace 10 occurs, and Qa+ΔQa is increased by this pressure difference. Therefore, in the case of fulfilling the equation (2), it is necessary to finely adjust ΔQa and ΔQb.
The introduction of the adhesion-preventing air into the furnace 10 has the effect of lowering a temperature in the furnace 10. Therefore, when the particles do not adhere to the inner wall of the furnace 10, it is preferable to decrease Qa in quantity as long as the adhesion of particles does not occur in order to maintain the temperature range for normal operation.
Accordingly, when the particles do not adhere to the inner wall of the furnace 10, it is necessary to decrease Qa in quantity. However, in the case of decreasing Qa in quantity, a variation in the pressure in the furnace 10 and a change in Qb accompanying this occur in a similar way to the aforementioned case where Qa is increased.
As described above, it is difficult to always maintain a constant Q because the pressure in the furnace 10 varies due to the increased and decreased quantities of Qa and Qb. Even if adjustment of the rotational frequencies of the blowers 40a and 40b is attempted at the same time in accordance with the increased and decreased quantities of Qa and Qb, it is very difficult to make the right timing for increasing and decreasing quantities of Qa and Qb, and there is the problem in that the trapping efficiency of the cyclone 20 cannot be prevented from being lowered by the change in Q.
Meanwhile, it is common for the sphering apparatus to continuously operate during several days to several months, so the temperature and the pressure in the furnace 10 vary due to reasons such as the variation of external temperatures between day and night. As a result, there is problem in that the quality and the production efficiency of the particles are deteriorated.
[Patent Reference 1] Japanese Patent Publication No. 3331491